1. Field of the Invention
The present invention relates to an iris type light quantity adjusting device provided in a lens device of an image-taking apparatus such as a video camera or a digital still camera.
2. Description of the Related Art
Image-taking devices such as video cameras and digital still cameras that take moving images or still images using a solid-state image pickup device such as a CCD placed on a focus plane of an image-taking optical system have a lens device including a light quantity adjusting device for adjusting light quantity in image-taking. An “iris type light quantity adjusting device” is a type of such a light quantity adjusting device.
FIGS. 10(A) to (C) show a configuration of a conventional iris type light quantity adjusting device. FIG. 10(A) shows aperture blades and drive pins in an open state of the iris type light quantity adjusting device having six aperture blades. Reference numerals 101 to 106 denote six aperture blades having the same shape, and the aperture blades 101 to 106 are arranged to successively overlap in part from the aperture blade 101 counterclockwise in the drawing. That is, the blades are successively arranged such that the aperture blade 102 is placed on the aperture blade 101, the aperture blade 103 is placed on the aperture blade 102 . . . , and the aperture blade 106 is placed on the aperture blades 105 and 101. A portion of each aperture blade placed under another aperture blade is shown by a dotted line.
A shaft 110 provided on an unshown fixed plate fits into a hole part 101a provided in a base of the aperture blade 101, and a drive pin 101c formed on a drive ring (not shown) that can rotate around an optical axis fits into a slot part 101b provided in an intermediate portion of the aperture blade 101. The other aperture blades 102 to 106 are supported by the same structure.
The drive ring having the drive pin fitting into the slot rotates around the optical axis, so that the aperture blades interlock with each other and pivot around the shafts formed on the fixed plate outward and inward in a direction orthogonal to the optical axis (radial direction). Thus, as shown in FIGS. 10(A) and 10(B), a central opening formed by all the aperture blades 101 to 106 changes the shape thereof to adjust quantity of light passing through the opening.
Further rotating the drive ring from the state shown in FIG. 10(B), the aperture blades 101 to 106 overlap in order to completely close the central opening.
However, for smooth movement from the state shown in FIG. 10(B) to the state shown in FIG. 10(C), the six aperture blades 101 to 106 are required to keep correct overlapping order such that the aperture blade 102 having no overlapping relationship with the aperture blade 106 in the state of FIG. 10(B) gets under the aperture blade 106, and the aperture blade 101 having no overlapping relationship with the aperture blade 105 in the state of FIG. 10(B) gets under the aperture blade 105.
In FIG. 10(C), the aperture blade 106 is omitted for clarity of a relationship between the aperture blades 101 and 105.
If the aperture blade 102 gets on the aperture blade 106, or the aperture blade 101 gets on the aperture blade 105 when moving from the state of FIG. 10(B) to the state of FIG. 10(C), the order of the six aperture blades 101 to 106 changes to prevent complete closing of the central opening.
In moving image taking, movement of the aperture blades in the above described iris type light quantity adjusting device may follow variation in brightness of a subject and is thus relatively slow.
In shutter operation for taking still images by the iris type light quantity adjusting device in the video camera having a still image recording function or the digital still camera taking the still images, each aperture blade is driven as fast as possible for higher shutter speed.
However, the aperture blade is formed of a thin sheet material for reducing space and driving load, and thus has low rigidity and is light in weight. Therefore, when driven at high speed, the aperture blade may flutter toward the other aperture blades (in an optical axis direction) to tilt the tip thereof or catch air to be raised. In this case, a problem arises that the aperture blade gets on the other aperture blade to prevent complete closing of the opening.
Japanese Utility Model Laid-Open No. 2-48928 proposes aperture blades corresponding to the aperture blades 102 and 106 described with reference to FIGS. 10(A) to (C), a tip of the aperture blade 102 and a base of the aperture blade 106 being extended to overlap for ensuring overlapping order of the aperture blades, thereby preventing bite between the aperture blades.
However, the document has no reference to aperture blades corresponding to the aperture blades 101 and 105, and the risk of bite between the aperture blades still remains.
Even if the aperture blades 101 and 105 are also formed as proposed in Japanese Utility Model Laid-Open No. 2-48928, problems arise as described below with reference to FIGS. 11(A) and 11(B).
FIG. 11(A) shows aperture blades in an open state of an iris type light quantity adjusting device having six aperture blades. A portion of each aperture blade placed under another aperture blade is shown by a dotted line.
Aperture blades 201 and 202 are aperture blades with extended portions on the tip side, aperture blades 203 and 204 are normal aperture blades (without extended portions), and aperture blades 205 and 206 are aperture blades with extended portions on the base side so as to overlap the respective extended tips of the aperture blades 201 and 202.
In FIG. 11(B), the aperture blade 206 is omitted from FIG. 11(A) for clarity of a relationship between the aperture blades 201 and 205. As shown in FIGS. 11(A) and 11(B), the aperture blades 201 to 206 are arranged to successively overlap in part from the aperture blade 201 counterclockwise in the drawing. That is, the blades are successively arranged such that the aperture blade 202 is placed on the aperture blade 201, the aperture blade 203 is placed on the aperture blade 202, . . . , and the aperture blade 205 is placed on the aperture blade 204 and 201, and further, as shown in FIG. 11(A), the aperture blade 206 is placed on the aperture blades 205 and 202.
In the open state shown in FIG. 11(A), the tip of the aperture blade 202 gets under a base side portion of the aperture blade 206, and the tip of the aperture blade 201 gets under a base side portion of the aperture blade 205, thereby protecting the aperture blade 202 or 201 from getting on the aperture blade 206 or 205 to prevent complete closing.
However, the aperture blade 201 is placed in the lowermost position, thus, as shown in FIGS. 12(A) to 12(C), an extended tip 201a (FIG. 12(A)) originally placed in an upper side of a fixed opening 207 (inside in an optical axis direction) formed in an unshown fixed plate may extend, by the deflection thereof, toward a lower side of the fixed plate (outside in the optical axis direction) through the fixed opening 207 (FIG. 12(B)). In this case, while each aperture blade opens from a closed state, the tip 201a of the aperture blade 201 catches a periphery of the fixed opening 207 in the fixed plate to prevent opening of the aperture blade 201.
The aperture blade 202 is placed between the aperture blades 201 and 206, so that inconvenience as described above does not occur.
As shown in FIG. 12(C), the extended tip 201a of the aperture blade 201 overlaps the opposite aperture blade 204 at a closed position, the overlapping order thereof appearing only in a near-closed state. However, if the tip 201a of the aperture blade 201 gets on the aperture blade 204, complete closing is prevented.
A relationship between the tip of the aperture blade 202 and the aperture blade 205 is also the same, thus preventing complete closing by the aperture blade getting on the other aperture blade.